Estimation of symbiotic N2 fixation in an Amazon floodplain forest

Sustainable management for existing Amazonian forests requires an extensive knowledge about the limits of ecosystem nutrient cycles. Therefore, symbiotic nitrogen (N₂) fixation of legumes was investigated in a periodically flooded forest of the central Amazon floodplain (Várzea) over two hydrological cycles (20 months) using the ¹⁵N natural abundance method. No seasonal variation in ¹⁵N abundance (δ ¹⁵N values) in trees which would suggest differences in N₂ fixation rates between the terrestrial and the aquatic phase was found. Estimations of the percentage of N derived from atmosphere (%Ndfa) for the nodulated legumes with Neptunia oleracea on the one side and Teramnus volubilis on the other resulted in mean %Ndfa values between 9 and 66%, respectively. More than half of the nodulated legume species had %Ndfa values above 45%. These relatively high N gains are important for the nodulated legumes during the whole hydrological cycle. With a %Ndfa of 4–5% for the entire Várzea forest, N₂ fixation is important for the ecosystem and therefore, has to be taken into consideration for new sustainable land-use strategies in this area.